This may or may not be workable for your forum but for my phpBB forums I keep spammers from registering by... turning off registration

Instead I edited the registration pages to indicate that people who wanted to join the forums should send an appropriate email to a spambot-obscured email address such as zapkitty at no such address dot com

This way you get the email validation out of the way before you even register the new user and you don't have to carry a bunch of not-yet-purged spambots on your membership list.

Drawback: you have to monitor that email box as there's nothing from the forum software to flag that a potential new user is trying to get your attention.